[[UIApplication sharedApplication] openURL:[NSURL URLWithString:@"http://www.stackoverflow.com"]];
Will open a URL by the Application defined for the scheme. http has Safari as default (I guess)
To open it in the App itself create a ViewController + xib, add a UIWebView to your View & Buttons to get back to the app.
Then you can create an instance of your new ViewController and present it with
[self presentViewController:WebVC animated:YES completion:^(void){
[[WebVC MyWebView] loadRequest: [NSMutableURLRequest requestWithURL:[NSURL URLWithString: @"http://stackoverflow.com"]];
}];
e.g.
To make it even easier you could add a function to your ViewController like this one
-(void) LoadUrlFromString: (NSString *)url{
[self.MyWebView loadRequest: [NSMutableURLRequest requestWithURL:[NSURL URLWithString: url]]];
}
and then just do as before but call
[WebVC LoadUrlFromString:@"http://stackoverflow.com"];
on completition